Filter chain



Oct. 19,1926. 1,603,806 H. RIEGGER FILTER CHA-IN Filed August a, 1922 4 Sheets-Sheet 1 .Oct. 19 1926.

H. RIEGGER FILTER CHAIN Filed August a, 1922 4 Sheets-Sheet 2 Oct. 19 1926. 1,603,806

H. RIEGGER FILTER CHAIN Filed August. 1922 4 Sheets-Sheet :5

W 5 11/ J W v V I l f Y I W w I I I I v 2 l 0 amt/div Oct. 19 1926. v

' 1,603,806 .H. RIEGGER FILTER CHAIN Filed August 3, 1922 4 Sheets-Sheet 4 Patented Oct. 19, 1926.

UNITED STATES PATENT OFFICE.

HANS RIEGGER, 0F IBERLIN-PANKOW, GERMANY, AS SIGNOR TO SIEMENS & HALSKE,

AKTIENGESELLSCHAFT, OF SIEMENSSTADT, NEAR BERLIN, GERMANY, A CORPORA- TION 0F GERMANY.

Application filed August 8, 1922, Serial No.

It is generally known that currents of a predetermined frequency can be separated from currents of different frequencies by usingan electric oscillatory circuit which is tuned to the desired frequency. It responds to the desired frequency while all other frequencies are more or less strongly suppressed. If several of such oscillatory circuits are connected in series, the deviating frequencies will disappear more and more with every consecutive circuit and the de sired frequency will be more and more emphasized. Such electric oscillatory circuits connected in series are known in the art as filter chains.

Filter chains are used in connection with wireless telegraphy and telephony and further with high frequency line telegraphy and telephony. When thus applied, it is, however, not suflicient to make the system responsive strictly to one single frequency as the sending frequency cannot always be maintained accurately. Thefilter chains should, if theyare to answer these purposes, rather permit the passage of several frequencies within a predetermined frequency range. When using a single oscillatory circuit, this can be attained by a somewhat stronger damping; the greater the damping the flatter will be the resonance curve.

In order that the invention may be clearly undersood, I shall proceed to describe the same with reference to the accompanying drawings, in which:

Fig. 1 shows a resonance curve of the simplest form, characteristic of a damped circuit;

Fig. 2 shows an ideal resonance curve of rectangular form;

Fig. 3 shows the resonance curve of a five-link filter chain;

Fig. 4 shows the resonance curves of a chain with two oscillatory circuits and of a chain with three oscillatory circuits;

Fig. 5 shows the curve produced by the chain of fivelinks;

Fi 6 represents a circuit'diagram of a five-llnkchain referred to in Fig. 5;

Fig. 7 shows the damping curve and the damping and coupling curve of a cham, one part of which consists of one link and the other part ofv two links Fig. 8 is a diagramof aresonance curve FILTER CHAIN.

580,470, and in Germany August 31, 1921.

produced by the superposition of the two curves inFig. 7;

Fig. 9 shows the resonance curves of the different parts of an improved filter chain;

and

Fig. 10 shows the resonance curve of the entire chain.

In Fig. 1, I designates the resonance curve at smaller'damping and II the resonance curve at greater damping. As abscissac the frequencies, and as ordinates the amplitudes of current, are plotted. The oscillatory circuit is tuned to the period of vibrations (0 The requirement is that the frequencies within the range w--o" freely pass through the circuit, but that frequencies below or above that range should be excluded. The figure shows that an oscillatory circuit with the resonance curve II produced by increased damping is not a satisfactory expedient. For,- it will be observed that those frequencies' which correspond exactly to the desired frequency are considerably reduced in oscillation amplitude, whereas the undesired frequencies are not sufliciently suppressed. The ideal would be a resonance curve of rectangular form as shown in Fig. 2. With this curve, all frequencies between a and or would be transmitted equally well, but all other fre quencies would' be almost entirely suppressed.

While, of course, a resonance curve of this exact rectangular form would be very difficult to obtain, the means proposed by, the present invention result in a resonance curve, very closely approximating this ideal form.

If filter chains are used, the frequency range to be transmitted can be enlarged by providing a close coupling of the individual I that this curve is already much more similar to the rectangular curve shown in Fig.2 than the resonance curve II of Fig. 1. By

varying: the tuning, the damping of a highly damped circuit, and the degree of coupling of the different links of the filter chain, the shape of the resonance curve can should be altered, and it might even be possible to produce, by experiments, filter chains which in a satisfactory manner come close to the ideal form of Fig. 2. Such experiments require, however, much time and are' not very reliable, as they do not give any guarantee of attaining the best that can be realized with the means at disposal.

The present invention relates to a special kind of filter chain, in which, by calculation or by designing, a favourable form of the resonance curvecan be produced. The fact that the tuning frequencies, the damping, and the degrees of coupling, required for this purpose, can be determined before this chain is constructed, constitutes an important improvement over the filter chains known at present.

The filter chains, according to the present invention, are characterized by the feature that the degree of coupling between two of their links is so loose that between these two links no noticeable efiiect of coupling resonance is produced, while the coupling between all other links is closer, e. g. so close that distinct couplingrresonance points are produced. The; looserci'o'upling point separates thus the chain-'iainto two parts, each part by itself possessing its individual resonance curve. Each of the twocurves shows a number of peaks which are the more pronounced the closer the coupling is between the individual links of the corresponding part of the chain. The greater the damping is made at the same degree of cou pling, thecloser will the peaks come together. The number of and the distance between the peaks can be determined by calculation. The calculation becomes specially simple if each part per se of the chain is made homogenous, e. g. if all the links of the correspondin part of the chain are tuned to the'same requency and have the same damping andv if the different links possess the same degree of coupling. i Even if the decrements .do not agree ,a common average decrement can be used for the calculation, provided, of course, that the number of links is not too large. By loose coupling of the two parts of the chain, a resonance curve is obtained which resembles very nearly the rectangular curve, provided the peaks and the valleys of each part are selected in such a manner'that the peaks of one resonance curve are located between the peaks of the other resonance curve. This can be car-- ried out in a particularly easy manner if one part Ofzthe' chain has an even number of linksand the other part an odd number of links. In thiscase, a valley will occur at the middle of the first Chfilll part and a peak at;the middle. of the other chain part.

valleys of themther curve spurts. ispro- If, further, care is taken that the peaks of one curve conicide with the duced such as is shown for instance in Fig. 5. This curve results from the superposition of the two resonance curves III and IV of Fig. 4. III is the resonance curve of a chain with two oscillatory circuits, and IV that of a chain with three oscillatory circuits. The circuits are diagrammatically shown in Fi 6. The first part of the chain consists of the oscillatory circuits 1 and 2 which are closely coupled with each other and excited by the windings 6. The second part of the chain consists of the oscillatory circuits 3, 4, 5. The fifth oscillatory circuit 5 transmits the energy to the windings 7 whichconduct the same to any receiving device desired. The coupling between 1 and 2 is so close that a distinct two peak curve is produced such as shown at III in Fig. 4E. The oscillatory circuits 3, 4, 5 are also coupled with each other so closely that the effect of the coupling resonance is distinctly perceivable (curve IV, Fig. 4), although the formation of several peaks has not yet been attained. If now the first part of the chain and the second part are coupled with each other by loosely coupling the oscillatory circuits 2 and 3, the curves III and IV will coincide. In order to find out, as function.

[of the frequency, those current amplitudes whichat constant excitation are transmitted from the windings 6 to the windings 7, it is merely necessary to multiply with each other similar values of the curves III and IV. The

result will be the resonance curve shown in Fig. 5 of the entire chain consisting of five links. This curve comes already very near the rectan ular shape. The slight fluctuations in the horizontal part of the curve between a and d are not at all disturbing so that oscillations of all frequencies which are situated between these two frequencies a and a, pass e ually freely through the chain. In front 0 point 0 and behind point d the curve drops everywhere quite steeply.

Owing to the ease with which the curve in Fig. 5 can be calculated from the curves III and IV (by multiplication of the corresponding values of III and IV) it is not difficult to select, in designing, those forms ofthe curves III and IV which together result in the most favourable compound curve. The forms for the curves III and IV on the other hand can easily hevaried, as mentioned above, by altering the damping and the coupling degrees.

The advantages of the invention become already obvious if one part of the chain consist of one link only and the; other part of the chain of two links. If the damping for one link is so selected that the curve V of Fig. is produced, and if the dam ing and thef'coupling' degree of the two lin of. the other part are so selected that the resonance curve VI is produced, both curves together Fig. 8, which, obviously, is a really very favourable curve for practical purposes.

The resonance curve for the entire chain can be found from the resonance curves of its parts by a simple multiplication of the corresponding values, but only if the couit will frequently be possible to further im- I prove the form of the curve by a slight increase in the coupling factor between the two parts. Fig. 9 shows the resonance curves VII and VIII of the individual parts of a chain by the superposition of which portions the resonance curve of the entire chain shown in Fig. 10 is produced. The curve portion of Fig. 10 drawn, in full hnes is. the resonance curve with loose couplmg between the two parts of the chain. The figure shows that the two lateral peaks e and f are considerablyhigher than the middle peak g. If the damping and cou hng for the individual parts of the chain ave been determined in such a manner that the full line curve shown in Fig. '10 is produced, the peaks 6 and f can now also be lowered so far that they do not much exceed the peak 9, as is shown in dotted lines in Fi 10. This result is attained by somewhat c oser coupling between the two parts of the chain. If this coupling is selected sufliciently close, the effect of coupling resonance will "ust be noticeable, the peaks of the curve in ig. 10 being then no longer sim le products rom the curves VII and VII of Fig. 9, but in this case a complicated calculation takes the place of a simple multiplication. Anyhow,

, one or at the utmost two experiments will be pling, which brings the three peaks suflicient to ascertain that degree of qoua 9 approximately to the same height. The coupling degree between thetwo parts of the chain must, however, still be smaller than the coupling degrees between thelinks of each individual part, as otherwise the less perfect curve of an ordinary five-link chain as shown in Fig. 3 would be produced.

It is obvious to any skilled person that a third chain part could be added by loose coupling to a chain composed of two loosely Y coupled parts in order to still closer approximate the rectangular form by superposition of a third resonance curve.

What I claim, is:

1. Filter chain for transmitting oscillations within a limited range of frequencies, comprising at least two coupled groups of oscillatory circuits, at least one of said groups comprising at least two adjacent oscillatory circuits, coupled with each other suificiently close to produce a resonance curve of that group having at least two peaks, the resonance peaks of one group of circuits occurring at frequencies different from those at which the peaks of the other group occur.

2. Filter chain for transmitting oscillations within a limited range of frequencies comprising at least two coupled groups of oscil atory circuits, at least one of said groups comprising at least two adjacent oscillatory circuits, coupled with each other sufliciently close to produce a resonance curve of that group having at least two peaks, the resonance peaks of one group of circuits coinciding wlth the resonance valleys of the other group.

3. Filter chain for transmitting oscillations within a limited range of frequencies,

comprising at least two coupled groups of oscillatory circuits, at vleast one of said groups comprising at least two adjacent .oscillatory circuits, coupled with each other sufliciently close to produce a resonance curve of that group having at least two peaks, one of said groups having an even; number and the other group having an odd number of oscillatory circuits.

4. A filter chain for transmitting oscilla-v tions within a limited range of frequencies comprising at least two groups of osclllatory circuits, eachof said two groups comprising at least two adjacent oscillatory circuitscoupled with each other sufiiciently closeto produce a resonance curve group having at least two peaks, and means for loosely coupling said groups with each other.

5. A filter chain according to claim 4 in which the resonance peaks of one group of circuits occur at frequencies different from those'at which the peaks of the other group occur.

6. A filter, chain according to claim. 4 in which the resonance peaks of one group of circuits substantially coincide with the resonance valleys of the other group.

7. A filter chain according to claim 4 in for the respective which the circuits comprising each group are tuned to the'same frequency.

HANS RIEGGER. 

